1. Field of the Invention
The present invention relates to a signal processing apparatus and method for an optical disk system.
2. Description of the Related Art
An optical pickup is an apparatus that performs record/playback of information on/from optical disks by using an optical system that is a combination of a light source, a lens, a photo-detector, and the like. A signal obtained by this optical pickup from the optical disk (hereinafter, called a light-detected signal) varies in level due to variations in characteristics of optical disks such as reflectivity, variation in the manufacture of the optical system including the photo-detector of the optical pickup, variation in the amount of light depending on an operation mode, either a record, playback, or erase, and the like. Accordingly, optical disk systems to perform record/playback of optical disks are usually provided with an AGC (Auto Gain Control) circuit to make the level of the light-detected signal coincide with a predetermined reference level. See, for example, Japanese Patent No. 3272003.
FIG. 15 is a diagram showing an optical disk system having a conventional AGC circuit 40. Information recorded on an optical disk 10 is read out by an optical pickup 20 in the form of returned laser light, and is converted to electricity to obtain a light-detected signal. This light-detected signal is a signal on which a DSP 50 performs various optical disk playback processes (such as decoding and a servo process). An RP signal, an FE (Focus Error) signal, a TE (Tracking Error) signal, and the like are produced from the light-detected signal.
Note that because the light-detected signal is minute in level at the stage of being detected by the optical pickup 20, a preamplifier 30 amplifies the light-detected signal to a level that can be handled by the DSP (Digital Signal Processor) 50 and the like at the later stage. And, the light-detected signal amplified by the preamplifier 30 is supplied to the AGC circuit 40 to accommodate to the level variation. As a result, the light-detected signal stabilized at an almost constant level is supplied to the DSP 50, and after decoded according to the standard of the optical disk 10, is outputted as a playback signal (an audio signal, a CD-ROM signal, a video signal, etc.) to a speaker 60 and the like.
The AGC circuit 40 is constituted by an analog circuit comprising a VCA (Voltage Control Amplifier) 41 that amplifies or attenuates the light-detected signal with a variable gain according to a control voltage, a detector 42 for detecting the output of the VCA 41, and a VCA control circuit 43 that generates and supplies the control voltage to make the output of the VCA 41 detected by the detector 42 coincide with the predetermined reference level to the VCA 41. An internally generated voltage obtained by dividing a supply voltage VCC according to a predetermined ratio is usually used as the reference level in the VCA control circuit 43.
However, with a conventional mechanism for stabilizing the level of the light-detected signal like the AGC circuit 40 of FIG. 15, various noise factors such as variations in characteristics of the optical disk and the optical pickup depending on environmental conditions and the like, and variations in the amount of reflected light due to stains or scratches on the optical disk may cause unexpected, minute variation in the level of the light-detected signal.
In the conventional mechanism, the stabilization reference for the light-detected signal is set at a fixed level by an analog component such as a voltage divider. Hence, the conventional mechanism is not flexible enough to deal with the unexpected, minute variation in the level of the light-detected signal, but allows the level control to follow the minute level variation. As such, with the conventional mechanism, it is difficult for the light-detected signal to settle in level, and thus the problem occurs that optical disk playback processing cannot be appropriately performed on the light-detected signal.